UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Television and Video

Notices

Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

Closed Thread
 
Thread Tools
Old 5th Nov 2019, 3:50 pm   #21
Sideband
Dekatron
 
Sideband's Avatar
 
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,548
Default Re: GEC Junior fineline picture problem.

If the line speed is wrong, it might affect the field amplitude....Most sets have the field supply derived from the line transformer so if line speed is wrong, it could lower the supplies from the transformer maybe enough to affect field amplitude. My gut feeling is to sort out the line speed first. Does the line hold control correct the line speed when adjusted? If so the oscillator is running OK. If you can't correct the line speed using the control then the oscillator is running at the wrong speed.

Suggest you also check the flywheel sync diodes. If either of these are faulty then it can be impossible to get the line speed correct.

Seems odd for two faults to appear at the same time....I suspect they are interlinked but I say again, get the line speed right first.
__________________
There are lots of brilliant keyboard players and then there is Rick Wakeman.....
Sideband is offline  
Old 5th Nov 2019, 9:35 pm   #22
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: GEC Junior fineline picture problem.

Quote:
Originally Posted by Sideband View Post
If the line speed is wrong, it might affect the field amplitude....Most sets have the field supply derived from the line transformer so if line speed is wrong
Interesting remark. I have seen it in a valve based TV and color sets, but I have never seen the vertical oscillator or vertical amplifier in a transistorized monochrome TV set or video monitor powered by any output from the line output stage, they usually share a low voltage power supply in the range of 12 to 30V.

Also as you see from the OP's schematic, the vertical amplifier and oscillator are powered by the same power supply as the line stage, the only difference being a series filter choke and a filter capacitor feeding the V stages.

It is unlikely that voltage will be low to the V stages alone as chokes generally are ok or go open, but its not impossible for the resistance of the choke to elevate if there is a high resistance solder join. And we know the supply voltage to the line stage must be correct as the scanning width is correct and by implication the line frequency close to correct, albeit not locked.

It probably doesn't matter which fault (the line lock or reduced V scan amplitude) is hunted down first, as they appeared at the same time and probably have one cause.

But the V amplitude fault is severe and the line hold subtle, so I would target the cause of the former first and the latter second as in theory at least, it will be easier to determine, but it will result in a successful repair either way, most likely.
Argus25 is offline  
Old 6th Nov 2019, 11:00 am   #23
Hybrid tellies
Nonode
 
Hybrid tellies's Avatar
 
Join Date: Jan 2004
Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area.
Posts: 2,965
Default Re: GEC Junior fineline picture problem.

I would be looking for a common denominator here which on these type of sets is the power supply before delving into the line oscillator, frame osc/output or sync circuits.
I have gone through all the responses and as far as I can tell you have not confirmed to us what you are getting out of the PSU, ie should be about 11.6v. If this is out and there is a big ripple on the line as well you need to be checking for a PSU fault.
On these I have had that zenor diode in the regulator circuit fail, the main smoothing capacitor on the feed to the regulator and the diodes in bridge rectifier all fail giving weird and wonderful symptoms.
If the bridge rectifier or main smoothing capacitor are faulty you might be able to hear a louder than normal hum from the loudspeaker with the volume control turned right down.
__________________
Simon
BVWS member
Hybrid tellies is offline  
Old 6th Nov 2019, 11:19 am   #24
dazzlevision
Dekatron
 
Join Date: Sep 2003
Location: Near Swindon, North Wiltshire, UK.
Posts: 3,595
Default Re: GEC Junior fineline picture problem.

Quote:
Originally Posted by Hybrid tellies View Post
I would be looking for a common denominator here which on these type of sets is the power supply before delving into the line oscillator, frame osc/output or sync circuits.
I entirely agree. You should confirm the power supply is functioning correctly first. That is easily done, including ac ripple level.
dazzlevision is online now  
Old 6th Nov 2019, 11:21 am   #25
GreasedMonkey
Pentode
 
GreasedMonkey's Avatar
 
Join Date: Feb 2019
Location: Worcester, Worcestershire, UK.
Posts: 183
Default Re: GEC Junior fineline picture problem.

Thanks for the replies all. I have not had change to go back to the set yet, but I will try to answer some of the points raised thus far.

Hybrid tellies, I no longer have the bridge rectifier in place as one of the diodes in this failed so I made a bridge out of 4 separate rectifier diodes. I have not measured the supplies yet as a previous post (I think by Argus25) suggested this would not be a supply issue but I will check them next time I am in there. I have however powered it up from a bench supply 12V dc so bypassing the front end and symptoms still the same.

Sideband, Slidertogrid, Line speed seems about right. I get a nice sawtooth wave, the frequency of which does change with adjustment of the H-Hold control so I assume the line oscillator is working as it should. I can obtain a sawtooth with a period of 64uS so am happy with that.

Argus25, I have not had time to go back and check frequency of the frame oscillator or decoupling capacitors yet.

My next step is to check the supplies for voltage and excessive ripple and the frame oscillator frequency and decoupling capacitors as suggested by Argus25. I will report back my findings. I am reasonably happy that the line oscillator is running correctly at this stage.
With help from this forum I have resurrected this set once, I am sure I can do it again with all the knowledge contained in here.
GreasedMonkey is offline  
Old 6th Nov 2019, 11:26 am   #26
Welsh Anorak
Dekatron
 
Welsh Anorak's Avatar
 
Join Date: Apr 2005
Location: North Wales, UK.
Posts: 6,884
Default Re: GEC Junior fineline picture problem.

From what you say above, I assume you can tune through line lock with the hold control? This definitely points to a sync fault which would explain the field timebase being unhappy. This TV doesn't use line derived power to the frame stage, but as you've run it with an external 12v supply HT problems can be ruled out. As far as I remember this TV used Lockfit transistors which are known for causing us unnecessary headaches!
__________________
Glyn
www.gdelectronics.wales
Welsh Anorak is online now  
Old 6th Nov 2019, 11:40 am   #27
GreasedMonkey
Pentode
 
GreasedMonkey's Avatar
 
Join Date: Feb 2019
Location: Worcester, Worcestershire, UK.
Posts: 183
Default Re: GEC Junior fineline picture problem.

Its full of them. Lockfit transistors all over the place !
GreasedMonkey is offline  
Old 9th Dec 2019, 2:54 pm   #28
GreasedMonkey
Pentode
 
GreasedMonkey's Avatar
 
Join Date: Feb 2019
Location: Worcester, Worcestershire, UK.
Posts: 183
Default Re: GEC Junior fineline picture problem.

So, Its been a while, but I have finally got around to investigating further with this problem.

I have looked at the waveforms given in the service info and found WF8 to be very low, around 25V where the book states 65V. This is the wave given to the vertical scan coil. Waveforms WF6 and WF7 seem OK so obviously a problem around that area.

Attached is a waveform page from the service info and the schematic, marked up with the locations of these waveforms. Also the waveforms as seen on the scope.

At this point I ran out of time so did not get into investigating in more depth. As usual, any suggestions welcome.

Thanks all.
Attached Thumbnails
Click image for larger version

Name:	WF1.jpg
Views:	80
Size:	37.6 KB
ID:	195078   Click image for larger version

Name:	WF2.jpg
Views:	64
Size:	38.0 KB
ID:	195079   Click image for larger version

Name:	WF3.jpg
Views:	65
Size:	38.2 KB
ID:	195080  
Attached Files
File Type: pdf Markup Schematic.pdf (1.02 MB, 70 views)
File Type: pdf Waveform list.pdf (376.7 KB, 65 views)
GreasedMonkey is offline  
Old 9th Dec 2019, 2:56 pm   #29
GreasedMonkey
Pentode
 
GreasedMonkey's Avatar
 
Join Date: Feb 2019
Location: Worcester, Worcestershire, UK.
Posts: 183
Default Re: GEC Junior fineline picture problem.

Remaining waveforms WF4 to WF8.

All waveforms were captured using X1 probe other than WF8 which was using X10 probe.
Attached Thumbnails
Click image for larger version

Name:	WF4.jpg
Views:	65
Size:	35.4 KB
ID:	195081   Click image for larger version

Name:	WF5.jpg
Views:	55
Size:	38.7 KB
ID:	195082   Click image for larger version

Name:	WF6.jpg
Views:	58
Size:	36.6 KB
ID:	195083   Click image for larger version

Name:	WF7.jpg
Views:	55
Size:	37.8 KB
ID:	195084   Click image for larger version

Name:	WF8.jpg
Views:	58
Size:	41.1 KB
ID:	195085  

GreasedMonkey is offline  
Old 9th Dec 2019, 6:13 pm   #30
thermionic
Heptode
 
thermionic's Avatar
 
Join Date: Apr 2003
Location: Cornwall, UK.
Posts: 985
Default Re: GEC Junior fineline picture problem.

...........great, but what does the main 11 Volt rail measure on the Collector of TR 401?

If that is ok, then check for 100 Volts at C234. If low, suspect the 10uF electrolytic.


Cheers. SimonT.
__________________
The honesty of imperfection..........

Last edited by thermionic; 9th Dec 2019 at 6:19 pm.
thermionic is offline  
Old 9th Dec 2019, 7:28 pm   #31
GreasedMonkey
Pentode
 
GreasedMonkey's Avatar
 
Join Date: Feb 2019
Location: Worcester, Worcestershire, UK.
Posts: 183
Default Re: GEC Junior fineline picture problem.

Thanks Simon.

11v rail is healthy.

I had suspected an electrolytic as I know they don’t age well.

My suspicion was C215, but I assume you are referring to C234 as the main suspect.

Next job is to search through the components box and see if I have one, or need to ‘borrow’ one from the lab at work. The trouble is, through hole components are few and far between at work these days. There is a good selection of surface mount stuff, but more and more of what we make is going to embedded components, things like passives and Transistors are now part of the PCB itself and not actual components any more. Good for EMC and reliability, but not so good for scrounging the odd component.
GreasedMonkey is offline  
Old 9th Dec 2019, 8:08 pm   #32
thermionic
Heptode
 
thermionic's Avatar
 
Join Date: Apr 2003
Location: Cornwall, UK.
Posts: 985
Default Re: GEC Junior fineline picture problem.

Hi. As this 10uF electrolytic is subject to pulses of 15.625 KHz, then high ESR of this component will effect the efficiency of the smoothing. Therefore it’s more important to measure the ESR, rather than its capacitance in this situation.

I might be miles off, or course....
__________________
The honesty of imperfection..........

Last edited by thermionic; 9th Dec 2019 at 8:20 pm.
thermionic is offline  
Old 9th Dec 2019, 9:49 pm   #33
Sideband
Dekatron
 
Sideband's Avatar
 
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,548
Default Re: GEC Junior fineline picture problem.

Quote:
Originally Posted by thermionic View Post
Therefore it’s more important to measure the ESR, rather than its capacitance in this situation.

.....that's assuming you can measure the ESR....if you can't, don't rely on a simple multimeter switched to capacitance. Very often they will give incorrect readings with an electrolytic. Try substituting it with a new capacitor....don't be tempted to bridge the old one with a new component as a quick test.
__________________
There are lots of brilliant keyboard players and then there is Rick Wakeman.....
Sideband is offline  
Old 13th Dec 2019, 10:59 am   #34
Heatercathodeshort
Dekatron
 
Heatercathodeshort's Avatar
 
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
Default Re: GEC Junior fineline picture problem.

This is what appears to be a very difficult fault to identify without an experienced 'hands on' approach.
I think many possibilities could be eliminated within a short period with the correct fault tracing. It is very difficult to do this via words alone. Maybe I'm wrong? John.
Heatercathodeshort is offline  
Old 13th Dec 2019, 11:57 am   #35
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: GEC Junior fineline picture problem.

Has the thermistor been checked?

Lawrence.
ms660 is offline  
Old 16th Dec 2019, 10:54 am   #36
GreasedMonkey
Pentode
 
GreasedMonkey's Avatar
 
Join Date: Feb 2019
Location: Worcester, Worcestershire, UK.
Posts: 183
Default Re: GEC Junior fineline picture problem.

No, I have not checked the thermistor. I have not yet had the time to go back to it (busy time of year).

I have no details of what the thermistor should be (other than its ident of TH201). What sort of value should I get at room temperature?
GreasedMonkey is offline  
Old 16th Dec 2019, 12:00 pm   #37
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: GEC Junior fineline picture problem.

I wouldn't know specifically without having any data on it but I would think it would be quite low, eg less than 10 ohms.

Lawrence.
ms660 is offline  
Old 20th Dec 2019, 7:55 pm   #38
harry_w
Diode
 
Join Date: Aug 2019
Location: Sheffield, South Yorkshire, UK.
Posts: 5
Default Re: GEC Junior fineline picture problem.

just from my years of experience repairing tvs and knowing that the electrolytic capacitors lead a hard life in line derived voltage supplies i would have checked c237,c231 and c234 hoping for a short cut to finding the fault before delving further into the circuit.
harry.
harry_w is offline  
Old 3rd Jan 2020, 12:09 pm   #39
GreasedMonkey
Pentode
 
GreasedMonkey's Avatar
 
Join Date: Feb 2019
Location: Worcester, Worcestershire, UK.
Posts: 183
Default Re: GEC Junior fineline picture problem.

Thanks all for the help and suggestions so far. Unfortunately I have been unable to pick this back up (literally) due to slipping a disc in my back. I am (gingerly) back to life now, so hopefully soon I will get back to it.
GreasedMonkey is offline  
Old 18th Jan 2020, 3:10 pm   #40
GreasedMonkey
Pentode
 
GreasedMonkey's Avatar
 
Join Date: Feb 2019
Location: Worcester, Worcestershire, UK.
Posts: 183
Default Re: GEC Junior fineline picture problem.

Hi all,

I have had an hour or so with this set again today. I will try to answer all suggestions in this post.

Lawrence, I have checked the thermister and it measures 1.8 ohm DC resistance.

SimonT, Check for 100V at C234 gives me 92V. As Thermionic states ESR is more important than value, I have replaced C234 with new component.

Harry W, C237 and C235 replaced with new components.

The situation following all of this is unchanged, I still have only around half of the required vertical deflection.
GreasedMonkey is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 12:20 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.